This invention relates to a downhole apparatus, and in particular but not exclusively to forms of expandable tubing and to forms of expandable filters and filter supports.
WO93/25800 (Shell Internationale Research Maatschappij B.V.) described a method of completing an uncased section of borehole. A slotted liner provided with overlapping longitudinal slots is fixed in the borehole and a tapering expansion mandrel is pushed or pulled through the liner. The liner is expanded by the mandrel to support the adjacent borehole wall.
WO97/17524 (Shell Internationale Research Maatschappij B.V.) describes a deformable well screen and method for its installation utilising two sections of concentric slotted tubing, such as described in WO 93/25800, with a series of circumferentially scaled filter segments therebetween. The screen is expanded by pushing or pulling an expansion mandrel through the screen.
The expansion mechanism of these arrangements is such that there is an axial retraction of the tubing on radial expansion. This not only creates difficulties in accurately locating and securing the ends of the tubing in a bore relative to adjacent tubing sections, but also may result in undesirable relative axial movement between the tubing and other elements mounted thereon, such as filter segments. Further, in such a filter arrangement, the radial expansion forces which must be applied to the outer section of expandable tubing are transferred via the filter medium or media located between the tubing sections; this limits the range of media which may be utilised in such arrangements to filter materials and configurations which will withstand significant compressive forces, in addition to the significant shear forces which the filter material will experience during expansion of the tubing sections.
It is among the objectives of embodiments of aspects of the invention to provide alternative expandable tubing forms, including expandable filters and filter supports, which overcome such disadvantages.
According to the present invention there is provided expandable tubing having a tubing wall comprising a plurality of deformable tubular structures, at least some of the structures having permeable walls and containing a filter medium such that fluid may flow through the structures and thus through the tubing wall.
This aspect of the invention is useful as a downhole filter or sand screen, the deformable tubular structures forming the wall of the tubing facilitating expansion of the tubing, and the tubular structures potentially serving as filter elements and also accommodating a selected filter medium or media. Also, the use of the tubular structures to accommodate or facilitate expansion assists in avoiding the longitudinal contraction which tends to occur on radial expansion of tubing defining overlapping longitudinally extending slots.
The tubular structures may extend longitudinally, helically, or in be positioned in any appropriate orientation. A substantially axial orientation may offer more straightforward assembly and resistance to bending, however for other applications a helical arrangement may offer greater flexibility and resistance to radial compressive forces.
The tubular structures may be of any material, structure or form which provides the desired degree of deformability, permeability and the desired degree of structural strength. In one embodiment, the tubular structures are of sintered ductile metal, while in other embodiments drilled or slotted tubes may be utilised. If sintered metal, or some other porous material of similar structure, is utilised to form the tubular structures, the pores of the material may be initially filled or occupied by another material to create an impermeable structure. This filling material may be subsequently removed, for example by application of an appropriate solvent, which may be produced fluid, or exposure to elevated temperature as experienced in deeper bores.
The tubular structures may be connected to one another by any appropriate method, for example metal structures may be welded or brazed to one another, or the structures may be retained between two expandable sleeves or tubes.
In other embodiments, the tubular structures may be defined by appropriately shaped sheets or elements, or unitary structures, for example two corrugated sheets or tubes which have been welded or otherwise secured together, or by extruding or otherwise forming the tubing wall in a form which incorporates tubular structures. These embodiments may form other aspects of the invention, in which the tubular structures are impermeable, that is fluid is prevented from flowing through the tubing wall, in one or both of the unexpanded and expanded configurations.
The tubular structures may feature substantially continuous walls, or may have discontinuities therein, for example the tubular structures may be substantially C-shaped.
The tubular structures may accommodate a filter medium of media, such as woven wire, porous foam, wire mesh or wire wool, or indeed any medium presently utilised as a filter and which could be located within a tubular structure and withstand the change in shape experienced by the tubular structures during expansion. Alternatively or in addition, the tubular structures may be lined with a filter media in the form of a flexible or deformable porous material.
The aperture or pore size defined by the tubular structures or the filter media therein may be selected as appropriate, depending on the intended application of the tubing: the tubing may provide a relatively coarse filter, for preventing passage of relatively large solids, or may be such that passage of liquid or very fine solids is prevented or restricted, and only passage of gas is permitted, by use of a tubular structure-lining material such an expanded PTFE, as produced under the Gore-Tex trade mark by W. L. Gore and Associates.